The present invention pertains to fibers and films prepared from polyesters of a dicarboxylic acid and a diol selected from 1,3-cyclohexanedimethanol, 1,4-cyclohexane-dimethanol, or 1,3-propanediol. The fibers and films contain an epoxide additive and, optionally, an antioxidant, and show an increased dry heat stability when exposed to elevated temperatures in the presence of air. The fibers and films are particularly useful in applications requiring elevated temperatures.
Polyester materials are widely used as extrusion and molding resins for applications such as fibers, films, automotive parts, food packaging, beverage containers, and the like. Commonly used polyesters include poly(ethylene terephthalate), herein abbreviated as xe2x80x9cPETxe2x80x9d, poly(1,4-butylene terephthalate), herein abbreviated as xe2x80x9cPBTxe2x80x9d, poly(1,3-propylene terephthalate), herein abbreviated as xe2x80x9cPTTxe2x80x9d, and poly(1,4-cyclohexylenedimethylene terephthalate), herein abbreviated as xe2x80x9cPCTxe2x80x9d. PET polymers are widely used because of their availability and modest price. PET polymers have relatively good heat resistance in dry conditions but tend to degrade readily under moist conditions at elevated temperatures. Similarly, PET fibers, yarns, and fabrics are known to exhibit excellent dimensional stability, that is, low shrinkage or growth during service. Although PET has a high resistance to thermal degradation, PET fibers can exhibit loss of tensile strength under continuous exposure to elevated temperatures. Because of these limitations, PET polymers have limited usefulness in certain durable and semi-durable applications which require elevated temperatures, such as, for example, paper making machine clothing, dryer felts, industrial belts, high temperature tape backing, filter media for hot air or liquid filtration, autoclavable products, and other applications involving extended exposure to steam or superheated steam.
Efforts to remedy this problem have been directed to producing a high molecular weight linear polyester having a low content of free carboxyl groups. One approach is to reduce the number of free carboxyls by the use of xe2x80x9cend-capping agentsxe2x80x9d, such as diazomethane, as disclosed by U.S. Pat. No. 3,051,212, or carbodiimides, as disclosed by U.S. Pat. Nos. 3,975,329 and 5,169,499, which react with the free carboxyl groups and thereby prevent their further reaction. These end-capping materials, however, are expensive, often toxic, and thus not suitable for commercial applications.
Polyester compositions containing epoxides are known (see, for example, U.S. Pat. Nos. 3,657,191; 3,627,867; 3,869,427; 4,016,142; 4,130,541; 4,115,350; 3,560,605; 4,374,960; and Japanese Kokai Patent Nos. 07166419; 55012871; and 54131695. The addition of epoxides to polyesters, however, can be problematic. For example, the epoxides may show a low reactivity which, in turn, can create processing and operational difficulties and give uncertain results. For example, U.S. Pat. No. 3,869,427 discloses that styrene oxide appears to increase the level of carboxyl groups when added to molten PET and that diepoxides are xe2x80x9cunsuitable for the formation of fibersxe2x80x9d. Various polyester blends and composites in combination with epoxides also are described, for example in U.S. Pat. Nos. 4,348,500 and 4,222,928. Such blends, however, are expensive and can require complicated processing. In addition, reinforced polymer composites are not suitable for the preparation of fibers. Thus, the compositions described in the above references do not adequately address the inherent susceptibility of PET to hydrolysis or provide compositions which show excellent stability to high temperatures under both dry and moist conditions.
In contrast to PET, polyester polymers based on poly(1,4-cyclohexylene-dimethylene terephthalate), i.e., xe2x80x9cPCTxe2x80x9d, have excellent high temperature hydrolytic stability, are essentially free of cyclic trimers and surface oligomers, and are well suited for filtration applications. PCT fibers have an inherent advantages, with melting points (abbreviated herein as xe2x80x9cTmxe2x80x9d) up to 290xc2x0 C., glass transition temperatures (abbreviated herein as xe2x80x9cTgxe2x80x9d) up to 90xc2x0 C., high resiliency, and softness in comparison with other polyester fibers. PCT polymers, however, are susceptible to oxidation and tend to degrade when heated at high temperatures in the presence of air. Although PCT polymers containing epoxides and antioxidants for molding and reinforced compositions also are known (see, for example, Minnick et al. in U.S. Pat. No. 5,428,086), the deficiencies of PCT polymers in dry-heat stability for fiber and film applications have not been addressed. We have now discovered that fibers and films prepared from PCT and PTT homo- and copolyesters melt-blended with multifunctional epoxides show excellent stability at elevated temperatures in the presence of air and under both moist and dry conditions. Our discovery is unexpected because fibers and films prepared from PET melt-blended with multifunctional epoxides show no improvement, and sometimes, a reduction in both dry and wet stability at elevated temperatures.
It has been found that certain polyester blends containing selected epoxide compounds and, optionally, antioxidants, such as hindered phenols and/or phosphites, have excellent elevated temperature stability in fiber and film form under both dry and moist conditions. Accordingly the present invention provides a polyester fiber comprising:
i) dicarboxylic acid residues comprising about 60 to 100 mole % of a first dicarboxylic acid residue selected from the group consisting of terephthalic acid, naphthalenedicarboxylic acids, cyclohexanedicarboxylic acids, and mixtures thereof; and from 0 to about 40% of a second dicarboxylic acid residue selected from the group consisting of aromatic dicarboxylic acids containing from about 8 to about 16 carbon atoms, aliphatic dicarboxylic acids containing from about 4 to about 16 carbon atoms, cycloaliphatic dicarboxylic acids containing from about 6 to about 16 carbon atoms, and mixtures thereof;
ii) diol residues comprising about 50 to 100 mole % of a first diol residue selected from the group consisting of 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, 1,3-propanediol, and mixtures thereof, and from 0 to about 50 mole % of a second diol residue selected from the group consisting of aliphatic diols containing from 2 to about 16 carbon atoms, cycloaliphatic diols containing from about 6 to about 16 carbon atoms, and mixtures thereof; and
iii) about 0.05 weight % (wt %) to about 5 wt %, based on the total weight of said polyester, of an epoxide additive comprising an epoxide compound with at least 2 epoxy groups per molecule.
The fiber compositions may take any of the following forms, namely monofilaments, multifilaments, tows, staple or cut fibers, staple yarns, cords, woven, tufted and knitted fabrics, nonwoven fabrics, including melt blown fabrics and spunbond fabrics, and multilayer nonwovens, laminates, and composites from such fibers. Because of their good resistance to degradation at elevated temperatures, our novel stabilized fibers are useful in dryer felts and other paper machine clothing, belting, filter media for hot air or hot liquid filtration, electrical, autoclavable, sterizable products, and other industrial applications.
The instant invention also provides stabilized polyester films. Thus, another embodiment of our invention is polyester film comprising:
i) a polyester comprising about 60 to 100 mole % of a dicarboxylic acid residue selected from the group consisting of terephthalic acid, naphthalenedicarboxylic acids, cyclohexanedicarboxylic acids, and mixtures thereof; about 50 to: 100 mole % of a diol residue selected from the group consisting of 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, 1,3-propanediol, and mixtures thereof; and
ii) about 0.05 weight % (wt %) to about 5 wt %, based on the total weight of said polyester, of an epoxide additive comprising an epoxide compound with at least 2 epoxy groups per molecule.
These films are useful for food, industrial, and medical packaging used under high temperature conditions, including membranes and supports for bioculture media.
Our invention also provides a process for stabilizing polyester fibers comprising:
i) melt blending a) a polyester comprising about 60 to 100 mole % of a dicarboxylic acid residue selected from the group consisting of terephthalic acid, naphthalenedicarboxylic acids, cyclohexanedicarboxylic acids, and mixtures thereof; about 50 to 100 mole % of a diol residue selected from the group consisting of 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, 1,3-propanediol, and mixtures thereof; b) about 0.05 wt % to about 5 wt %, based on the total weight of said polyester, of an epoxide additive comprising a epoxide compound with at least 2 epoxy groups per molecule; and
ii) melt spinning continuous strands of filaments.
The present invention thus provides polyester fibers and films with increased stability at elevated temperatures under both moist and dry conditions and in the presence of air. Our novel fibers and films may contain from 0 weight % to about 6 weight % (abbreviated hereinafter as xe2x80x9cwt %xe2x80x9d), based on the total weight of the polyester, of a hindered phenol, phosphorus-containing, or sulfur-containing antioxidant to provide additional stability. Our invention may be used to prepare a range of fiber types including monofilament, multifilament, yarns, bicomponent fibers, spunbond fabrics, and melt blown webs. The fibers may be readily produced in a range of sizes from about 2 micrometers (also referred to herein as xe2x80x9cmicronsxe2x80x9d and abbreviated as xe2x80x9cxcexcmxe2x80x9d) in melt blown webs, about 0.5 to about 50 d/f for staple fibers, and up to about 5000 d/f for monofilament. The fibers of our invention may also be used in crimped and uncrimped form. Our novel fibers and films thus solve many of the known dry heat and hydrolytic stability problems encountered with PET and other polyester materials and provide longer lived, more economical products.